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Galileo completes Io Flyby
Galileo completes Io Flyby
25 Nov 1999
(Source: Jet Propulsion Laboratory)

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NASA's Galileo spacecraft has completed the closest-ever encounter with Jupiter's volcanic moon Io, but not before giving ground controllers a Thanksgiving day white-knuckler.

The spacecraft dipped to the planned 300 kilometers (186 miles) above Io at 8:40 p.m. Pacific Standard Time on Thursday, November 25. Only four hours before the flyby, while Galileo was being bombarded by strong radiation near Io, its onboard computers reset and placed the spacecraft into standby mode. Onboard fault protection software told the spacecraft cameras and science instruments to stop taking data and enter a safe state until further instructions were received from the ground

. Galileo engineers at NASA's Jet Propulsion Laboratory sprang into action, scrambling to send new commands to the spacecraft and bring it out of safe mode in order to save the flyby.

"With so little time to spare, it would have been easy to think 'no way' can we do this," said Galileo project manager Jim Erickson. "But our team members jumped to the challenge, in some cases leaving behind half-eaten Thanksgiving dinners."

"We were prepared, because we knew this high-radiation Io flyby posed a risk to spacecraft components, and in fact we saw that the radiation caused some glitches during the October 10th Io flyby," Erickson said. "This planning paid off in a big way."

The team finished sending new computer commands to Galileo, which were received and executed by the spacecraft at 8:45 p.m. PST, four minutes after the closest approach to Io. This enabled the spacecraft to complete more than half of its planned observations of Io and its plasma torus (a doughnut-shaped region brimming with electrified particles), and all the planned observations of another Jovian moon, Europa. If all goes according to plan, the data will be transmitted to Earth over the next several weeks, and it will then undergo processing and analysis.

The October 10th Io flyby was performed at an altitude of 611 kilometers (380 miles). Pictures and other scientific information from that flyby provided fascinating new views of Io, which has more than 100 active volcanoes. Scientists hope that by learning more about volcanic activity on Io, we may learn more about volcanoes on Earth.

Additional information about the Galileo mission may be accessed at

Galileo has been orbiting Jupiter and its moons since December 1995. The spacecraft is approaching the end of its two-year extended Galileo Europa Mission, a follow-on to the primary mission that ended in December 1997. Because of the radiation risks to the spacecraft, the Io flybys were scheduled as a daring venture to cap the extended mission, after Galileo had already returned an immense amount of scientific information. JPL, a division of the California Institute of Technology in Pasadena, California, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

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Last Updated: 5 Jun 2001